1. Field of the Invention
This invention relates to the use of crosslinked thermoplastic polyurethane elastomeric materials as a coating for biomedical devices, more particularly to the use of such materials as a coating surrounding implantable biomedical devices, and more specifically the electrical leads attached thereto.
2. Description of Related Art
The use of thermoplastic polyurethane (TPU) and other synthetic materials which resist biodegradability to coat the electrical leads of heart pacemakers and other implantable biomedical devices is well known. Typically, a heart pacemaker is implanted surgically into the body and wires are extended from the electrical pacing device to the heart muscle. Amongst synthetically produced materials, polyurethane possesses an attribute of relative biocompatibility. Simply stated, polyurethane resists both post-implantation tissue rejection, as well as the degradation caused by contact with animal tissues and fluids, to a greater extent than most other polymers, except arguably silicone rubber.
However, polyurethanes are not totally biocompatible and exhibit certain disadvantageous degradations which become evident over time. These degradations can include hydrolytic degradations wherein mechanical properties such as tensile strength, elongation and elasticity are affected. In fact, many polyurethanes will decompose completely from hydrolytic degradation as a result of prolonged implantation. Another form of degradation consists of environmental stress cracking. Environmental stress cracking results in the generation of crazes or cracks in the polyurethane elastomer which are themselves produced by the combined interaction of a medium capable of acting on the elastomer, in this case blood and other bodily fluids, and a stress level above a specific threshold. Yet another form of degradation is that of metal ion induced oxidation (MIO). MIO is distinguished by the accelerated degradation of polyurethane elastomers resulting from contact with metal ions used alone or as alloys in pacing lead conductors.
Regardless of the specific mechanism giving rise to the degradation, the end result is one of scission of the polyurethane chain with an attendant decrease in molecular weight and a loss of desireable properties, including that of biocompatibility. As a result, the functionality of the TPU coated pacemaker and especially the leads, over time may eventually fail thereby endangering the life of the patient. To avoid this possibility of failure the implanted pacemaker leads, are often preemptively surgically replaced on the order of every five to ten years. Any substantial increase in the lifetime of pacemaker leads would serve to minimize the unnecessary surgical trauma involved in pacemaker lead replacement. As mentioned, the use of thermoplastic TPU as insulation on pacemaker leads is known. In addition, attempts to develop polyurethanes and pacemaker leads with improved biocompatibility are disclosed in the patent literature. U.S. Pat. Nos. 4,875,308, 5,133,742, and 5,109,107 disclose the development of polyurethanes substantially free of ether linkages. U.S. Pat. No. 4,851,009 discloses the development of polyurethane pacemaker leads coextruded with ajacket covering of silicon rubber. Lastly, U.S. Pat. No. 5,419,921 discloses the development of polyurethane pacemaker leads jacketed with a thin layer of polycarbonate polyurethane. However, there is no teaching in these references suggesting a substantially positive effect on the long term biocompatibility of polyurethanes, in general, or of cardiac pacemakers in particular.
Accordingly, it is a primary object of the invention to provide a method for making an coated electrically insulating flexible wire or lead for use with cardiac pacemakers, defibrillators, or other biomedical devices exhibiting improved biostability and biocompatibility following in vivo implantation.
In accordance with the invention, there is provided an apparatus comprised of a biomedical device coated with a flexible insulator applied and crosslinked by actinic radiation.
The process of crosslinking stabilizes polymers in general as described in more detail in U.S. Pat. No. 5,900,444, incorporated herein by reference in its entirety. In particular, crosslinking stabilizes thermoplastic polyurethanes while increasing their heat, solvent, and environmental stress crack resistance. The use of crosslinked thermoplastic polyurethanes in place of uncrosslinked TPU offers the promise of a greater lifetime for biomedical devices in general used in in-vivo implantation, and pacemaker wires in particular. Wires or leads, and even whole devices can be coated or jacketed with TPU, subsequently or simultaneously radiation crosslinked to a dose between 1 and 100 MRads, and then utilized for implantation purposes. It is expected that such crosslinking of TPU will afford additional protection against the degrading effects of long term exposure to animal tissues and fluids. Therefore, in accordance with the invention, biomedical devices and the electrical leads attached thereto can be coated with thermoplastic polyurethane, a blend of different thermoplastic polyurethanes, a blend of thermoplastic polyurethanes and other polymers, such as a silicone polymer, or with polyurethane copolymerized with other moities such as silane. In addition, all of these coating materials may be suitably combined with a crosslinking agent to facilitate crosslinking of the TPU is the presence of radiation. This coating, whether containing a crosslinking agent or not, is then suitably converted through the application of radiation to produce a thermoset polyurethane exhibiting the attributes of both biostability and biocompatibility.